1. Field of the Invention
This invention relates to an optical system and an optical apparatus using the same, and is suitable, for example, for a video camera, a still video camera, a copying apparatus, etc.
2. Related Background Art
An optical system comprised of only a refracting lens is known as an image pickup optical system having a zooming function. In this optical system, a spherical or rotation-symmetrical aspherical refracting lens is disposed rotation-symmetrically with respect to the optical axis thereof.
There is also known the zooming technique of changing the imaging magnification (focal length) of an optical system by moving a plurality of reflecting surfaces constituting a mirror optical system relative to one another.
For example, U.S. Pat. No. 4,812,030 discloses a technique of affecting the focal length change of an optical system by changing the interval from a concave mirror 101 to a convex mirror 102 and the interval from the convex mirror 102 to an image surface 103 relative to each other in the construction of a Cassegrainian reflector shown in FIG. 17 of the accompanying drawings.
FIG. 18 of the accompanying drawings shows another embodiment disclosed in the aforementioned patent. In FIG. 18, an object light beam 128 from an object is incident on a first concave mirror 121, is reflected by the surface thereof, becomes a convergent light beam that travels toward the object side and is incident on a first convex mirror 122, is reflected toward the imaging plane side thereby and becomes a substantially parallel light beam incident on a second convex mirror 124, is reflected by the surface thereof and becomes a divergent light beam incident on a second concave mirror 125, is reflected thereby and becomes a convergent light beam, and is imaged on an image surface 127.
In this construction, the interval between the first concave mirror 121 and the first convex mirror 122 is change d and t he interval between the second convex mirror 124 and the second concave mirror 125 is changed to thereby effect zooming and change the focal length of the mirror optical system of the entire system.
Also, in U.S. Pat. No. 4,993,818, the image formed by the Cassegrainian reflector shown in FIG. 17 is secondary-imaged by another mirror optical system provided at the rear stage, and the imaging magnification of this secondary imaging mirror optical system is changed to thereby effect the focal length change of the entire optical system.
These optical systems of the reflection type have required a great number of constituent parts, and to obtain necessary optical performance, it has been necessary to assemble the respective optical parts with good accuracy. Particularly, the relative positional accuracy of the reflecting mirrors is severe and therefore, the adjustment of the position and angle of each reflecting mirror has been requisite.
As a method of solving this problem, there has heretofore been proposed, for example, a method of making the mirror systems into a block to thereby avoid the assembly error of the optical parts caused during the assembly thereof.
FIG. 19 of the accompanying drawings shows an embodiment of a reflecting optical system disclosed in Japanese Laid-Open Patent Application No. 8-292368. In FIG. 19, a light beam from an object passes through a first surface R1 of a stop, and the light beam enters a first optical element B1. In the first optical element B1, the light beam is refracted by a second surface R2, is reflected by a third surface R3 and a fourth surface R4, is refracted by a fifth surface R5, and emerges from the first optical element B1. At this time, the light beam is primary-imaged on an intermediate imaging plane near the fourth surface.
Next, the light beam enters a second optical element B2. In the second optical element B2, the light beam is refracted by a sixth surface R6, is reflected by a seventh surface R7 and an eighth surface R8, is refracted by a ninth surface R9 and emerges from the second optical element B2. At this time, a pupil is formed near the seventh surface R7 in the second optical element B2. The light beam which has emerged from the second optical element B2 is finally imaged on an image surface P (the image pickup surface of an image pickup medium such as a CCD).
In this embodiment, during focal length change, the first optical element B1 is once moved in Z plus direction from the wide angle end toward the telephoto end, and thereafter is moved in Z minus direction. The second optical element B2 is moved in Z minus direction from the wide angle end toward the telephoto end. The image surface P is not moved during focal length change. The interval between the first optical element B1 and the second optical element B2 is narrowed by the focal length change from the wide angle end toward the telephoto end, and the interval between the second optical element B2 and the image surface P is widened.
In this invention, use is made of a plurality of optical elements in which a plurality of curved surfaces and flat reflecting surfaces are formed integrally with one another, and the relative position of at least two of the plurality of optical elements is appropriately changed to effect zooming to thereby achieve the downsizing of the entire mirror optical system and yet alleviate the disposition accuracy (assembly accuracy) of the reflecting mirrors which is liable to be in the mirror optical system.
Also, by adopting a construction in which a stop is disposed on the side of the optical system which is most adjacent to the object and in this optical system, the object image is formed at least once, there is a reduction in the effective diameter of the optical system in spite of being a zoom optical system of the reflection type having a wide angle of view, appropriate reflective power is given to a plurality of reflecting surfaces constituting the optical elements, and the reflecting surfaces constituting each optical element are eccentrically disposed to thereby bend the optical path in the optical system into a desired shape and achieve the shortening of the full length of the optical system in a predetermined direction.
It is often the case with the prior-art optical system having only a refracting optical element that the entrance pupil lies deep in the optical system, and there has been the problem that the greater the interval to the entrance surface located most adjacent to the object side as viewed from the stop, the greater becomes the effective diameter of the light beam on the entrance surface with the enlargement of the angle of view.
Also, the mirror optical systems having the focal length changing function disclosed in the aforementioned U.S. Pat. Nos. 4,812,030 and 4,993,818 have both required a great number of constituent parts including a reflecting mirror and an imaging lens, and to obtain necessary optical performance, it has been necessary to assemble the respective optical parts with good accuracy.
Also, particularly the relative positional accuracy of the reflecting mirrors becomes severe and, therefore, it has been necessary to affect the adjustment of the position and angle of each reflecting mirror.
Also, the optical system proposed in the aforementioned Japanese Laid-Open Patent Application No. 8-292372 has a feature that the downsizing of the entire mirror optical system is achieved and yet the disposition accuracy (assembly accuracy) of the reflecting mirrors which is liable to be in the mirror optical system is alleviated. In the optical system of this patent application, the positions of the stop and the entrance pupil are the same and therefore, to make F number constant during zooming, it has been necessary to change the diameter of the stop. Also, if the F number is determined, the diameter of the stop is determined, and when the image size is small as in a still video camera, the diameter of the small stop necessarily becomes small.